Prior art direction finding methods include:                (i) A conventional electronic support measure typically utilizes a small sparse array (e.g., 16 elements) of broadband antennas (e.g., spirals) that are non-uniformly spaced, each equipped with a separate receiver to collect and demodulate incoming signals. By analyzing the relative amplitudes and phases of the received signals, their angles of arrival may be determined. This approach is described in “EW 103 Tactical Battlefield Communication Electronic Warfare” by David Adamy, published by Artech House. An advantage of the present technology is that it requires only a single receiver to obtain the angles of arrival, providing significant reduction in system cost.        (ii) Another approach to direction finding is based on the so-called Multiple Signal Classification (MUSIC) algorithm. This approach requires a more densely populated antenna array than the conventional technique described above, with a receiver behind each element. The signals received by all the elements are processed to determine the angles of arrival (see “Multiple Emitter location and Parameter Estimation”, R. O. Schmidt, IEEE Trans. Ant. Prop., vol. AP-34, No. 3, March 1986). The technology disclosed herein offers significant cost advantage since only one or two receivers are utilized, rather than requiring that a separate receiver be associated with each element.        
Prior art coded aperture beamforming technology includes:                (i) U.S. Pat. No. 5,940,029 describes obtaining radar data by sequentially switching between transmit and/or receive elements. This is closely related to a Synthetic Aperture Radar (SAR). This approach differs from the present technology because it requires a complex matrix of RF switches that is costly to implement; it only requires a set of N single bit modulators inserted between the antenna elements and the summation network in a receiving application.        (ii) U.S. Pat. No. 7,224,314 describes a reflectarray with each antenna element containing switching devices that vary the reflection impedances of the elements. By setting the switching devices to particular values one obtains a reasonably well focused beam when the reflectarray is illuminated by a source. Changing the switching devices allows one to steer the reflected beam. The present technology is different in that the different modulator states are changed sequentially, each providing a wide angle, low gain (i.e., unfocused) beam. For example, if the modulators are single bit (i.e., two state) phase shifters, such as 0/180 deg phase shifters, all of the N modules (for an N element array) are all changed to a particular set of states, with a different set of states for each measurement. The modulator states are chosen to provide wide angle, low gain beams to cover the entire field of view. Effective high gain beams (i.e., focused beams) are then obtained in signal processing after the data is gathered.        (iii) U.S. Pat. No. 6,266,010 describes an antenna array divided into four quadrants, with the output of each quadrant modulated by a 0/180 degree phase shifter. By setting the phase shifters in various states one may obtain antenna patters similar to those produced by a monopulse array. There is apparently no disclosure of sequentially collecting data and then obtaining bearing angles through digital manipulation of the data. The technology disclosed herein utilizes modulators behind each antenna element (not each quadrant) and, unlike this prior art, does not form the desired physical beams but forms beams synthetically after the data is collected.        
It is believed that no one has previously proposed RF digital beamforming by coding an antenna array aperture with single bit modulators. Conventional phased array antennas place multi-bit phase shifters behind each element to form sharp transmit or receive beam patterns for a single measurement through constructive and destructive interference of the element fields. On the other hand, the technology disclosed herein uses only single bit modulators (e.g., phase shifters) that do not form sharp beam patterns (i.e., “pencil beams”) during a single measurement. Effective sharp beam patterns are produced digitally after data collection using digital signal processing. An important characteristic of the present approach, and one that distinguishes it from conventional phased arrays, as that the codes (phase shifter states) are selected so that the resulting beam patterns fill the desired field of view more or less uniformly, without any directive (“pencil”) beams. Under these conditions, adding additional bits (beyond one bit) will not improve radar performance since single bit phase shifter control is sufficient to obtain a linearly independent set of measurements that may be “inverted” to estimate the antenna element signals and then processing these signals to produce digitally formed beams. A conventional phased array radar may be operated in this single-bit mode to obtain range, velocity, and bearing angle information in an acquisition period reduced by approximately a factor of N, where N is the number of beams within the field of view. This is possible because Code Aperture Radar (CAR) disclosed herein acquires information from all scatterers within the field of view from a single radar acquisition period (i.e., a single set of frequency sweeps of sufficient extent and duration to produce the desired range and velocity resolutions). In contrast, a conventional phased array must dwell for that same acquisition period (for the same velocity resolution) for each beam location, sequentially stepping through all N beam locations and therefore taking N times as long. The advantage of operating a conventional phased array radar in this mode is that information may be collected very quickly over the field of view and for any objects of interest the array may then be operated with multiple phase shifter bits to direct a beam at the object and improve the radar sensitivity.
An advantage of this technology is a significant simplification of the antenna array, resulting in reduction in cost and power dissipation, while still providing estimates of range, velocity, and bearing, and obtaining these estimates in a fraction of the time required by a conventional phased array to cover a wide field of view. Conventional phased array radar antennas contain substantial microwave electronics (e.g., multi-bit phase shifters and variable gain amplifiers) at each antenna element, resulting in very high cost and high power dissipation. Furthermore, a conventional phased array must obtain range and Doppler estimates for each beam position sequentially, resulting in a long acquisition time for high gain beams covering a wide field of view. The present technology acquires range, Doppler, and bearing estimates within the same time period as a single beam position of a conventional phased array radar, substantially reducing the total acquisition time.
Another advantage of this technology in a radar implementation is that transmitted RF energy is not required to be focused into a high gain beam, but instead may be radiated over a wide field of view. This produces a radar signal with a low probability of interception by electronic sensors.
Additionally, this technology provides the advantage of software reconfigurability since the beams are formed by digital computation (i.e., synthetically). By changing the parameters of the signal processing algorithm, beams pointing in any direction (within the field of view) with any beamwidth (within the limits set by diffraction) may be obtained from the same set of hardware.